Failure to properly repair damaged DNA can lead to abnormal growth and the development of diseases such as cancer. Thus, it is not surprising that many cancer-predisposition syndromes and tumors contain molecular defects in various DNA damage-response/cell cycle checkpoint genes including ATM (mutated in ataxia telangiectasia), BRCA1 (Breast cancer gene 1), p53, NBS1, and CHK2. Given the complexity of the cellular response to DNA damaging agents, there will undoubtedly be newly uncovered genes that participate in this process. Thus the identification of new players in DNA damage response pathways is an important aspect of cancer research at the molecular level. Our lab is interested in studying the mechanisms regulating the biological responses to DNA damage and uses experimental systems derived from both Xenopus and humans to achieve this goal. In particular, we have been focusing on the role of the 53BP1 protein in these processes. Our preliminary data indicates that 53BP1 possesses several properties that might be expected for proteins functioning in DNA-damage signaling pathways. This most notably includes y-IR-inducible phosphorylation and focus formation within the nucleus. Indeed, our preliminary data from both amphibian and human systems classifies 53BP1 as a new component of the DNA damage-response network. How 53BP1 functions in these pathways is unknown as very little information has been published to date on this protein. This proposal seeks to understand the functional role 53BP1 plays in DNA-damage response pathways and to clarify its relationship to p53. Valuable information concerning how cells respond to DNA damage is likely to be uncovered through the study of 53BP1. The specific aims of this proposal are: 1) What are the amino acid residues of h53BP1 that are phosphorylated in response to DNA damage as determined by using: a) in vitro techniques with ATM kinase; b) in vivo approaches including phosphospecific antibodies. 2) What is the functional significance and extent of h53BP1 phosphorylation with respect to: a) the identity of its in vivo kinases; b) the intra S-phase checkpoint; c) nuclear localization to sites of DNA damage; d) its ability to influence protein-protein interactions. 3) What is the functional role of X53BP1 in Xenopus systems with respect to: a) the ability to perform various DNA damage responses in cell-free extracts lacking X53BP1; b) an analysis of X53BP1/Xp53 complexes during development.